Rubber disc for a record player turntable

ABSTRACT

A rubber sheet for a turntable characterized by being a cured molding of a rubber composition, the cured molding having a hardness within the range from 30° as measured with an A-type rubber hardness tester to 15° as measured with an F-type rubber hardness tester and an impact resilience of not less than 40%. This rubber sheet effectively prevents the tone quality of reproduced sounds from deteriorating due to howling or external vibrations and permits reproducing source sounds faithfully.

TECHNICAL FIELD

This invention relates to a rubber sheet for the turntable of a recordplayer.

BACKGROUND ART

The rubber sheet for turntable is used to isolate a disc record fromacoustic outputs from a loudspeaker, vibrations of a record playeritself, etc., thereby preventing the occurrence of howling and thedeterioration of SN ratio due to vibrations of the disc record itselfduring its reproduction. The howling is a sort of oscillation phenomenadue to the feedback of acoustic outputs from a loudspeaker to a pickupand occurs due to the transmission of vibrations from the speaker to therecord player via the floor or rack where the record player is mounted,or due to the direct vibrations of the record player cabinet caused by asound pressure from the loudspeaker. It is needless to say that anynormal playback of the disc record cannot be effected when the howlingoccurs. Even where no howling takes place, the frequency response of thepickup output would be altered and local resonance and the like would becaused by such external vibrations, and the tone quality of reproducedsounds would be affected adversely by the increase of inter-modulationdistortion, the deterioration of transient phenomena, etc. The howlingtakes place mostly due to external vibrations from the floor or rackwhere the record player is mounted. The majority of such externalvibrations is of the order not more than 100 Hz and particularlyincludes a superlow frequency component of approximately 5 to 10 Hz. Toprovide for an effective inhibition of howling, a rubber sheet forturntable which exhibits excellent vibration-absorbing properties insuch superlow frequency region is desirable.

However, conventional rubber sheets for turntable which are made ofnatural rubber, isoprene rubber, chloroprene rubber, butyl rubber,styrene-butadiene rubber, butadiene rubber, silicone rubber or urethanerubber, or mixtures thereof are inferior in vibration-absorbingproperties, especially in the superlow frequency range, and are unableto protect effectively the tone quality of reproduced sounds fromdeteriorating due to howling or external vibrations.

DISCLOSURE OF INVENTION

The inventor has made extensive and thorouth researches on thedevelopment of a rubber sheet for turntable which effectively inhibitsthe tone quality of reproduced sounds from deteriorating due to howlingor external vibrations and permits reproduced sounds as faithful assource sounds themselves (for example, human voices themselves). As aresult, it has been found that the above described object can beachieved by a rubber sheet which is a sheet-like cured molding of arubber composition and has a hardness within the range from 30° asmeasured with an A-type rubber hardness tester to 15° as measured withan F-type rubber hardness tester and an impact resilience of not lessthan 40%.

In the foregoing, the A-type rubber hardness tester is a rubber hardnesstester provided in JIS K 6301-1969. Further, the F-type rubber hardnesstester means Asker F-type rubber hardness tester made by Kobunshi KeikiMfg. Co., Ltd., which is mainly used for measurements of hardness offoam rubber, urethane foam or the like. Although being solid rubber,some of the cured rubber in accordance with the present invention is toolow in hardness to measure by means of the A-type rubber hardness testerwhich is used in measuring hardness of conventional solid rubbers. Tothis end, the lower limit of hardness of the cured rubber in accordancewith the present invention is defined by the hardness value as measuredwith the F-type rubber hardness tester.

An intermediate hardness between the measuring range of the A-typerubber hardness tester and that of the F-type rubber hardness tester maybe measured with a C-type rubber hardness tester. The C-type rubberhardness tester as used herein means one provided in the Society ofRubber Industry Japan Standard SRIS-0101 and is used to measure suchintermediate hardnesses between the measuring ranges of the A-type andF-type rubber hardness testers. The C-type rubber hardness tester ismostly used to measure the hardness of sponge, soft rubber or the like.As the C-type rubber hardness tester, Asker C-type rubber hardnesstester made by Kobunshi Seiki Mfg. Co., Ltd. is used.

The hardness of the cured rubber in accordance with the presentinvention may be determined with any of the A-type rubber hardnesstester, the C-type rubber hardness tester and the F-type rubber hardnesstester, if the measurement is possible with any of these rubber hardnesstesters. However, if a hardness as measured with the A-type rubberhardness tester is not more than 1°, it is preferable to measure thehardness with either the C-type rubber hardness tester or the F-typerubber hardness tester because the measurement with the A-type rubberhardness tester accompanies increased errors. Similarly, if a hardnessas measured with the C-type rubber hardness tester is not more than 1°,it is preferable to measure the hardness with the F-type rubber hardnesstester. Furthermore, if a hardness as measured with the C-type rubberhardness tester is not less than 99°, it is preferable to measure thehardness with the A-type rubber hardness tester. Similarly, if ahardness as measured with the F-type rubber hardness tester is not lessthan 99°, it is preferable to measure the hardness with the C-typerubber hardness tester or the A-type rubber hardness tester. Thehardness values measured with the A-type rubber hardness tester, theC-type rubber hardness tester and the F-type rubber hardness tester willhereinafter be referred to as A hardness, C hardness and F hardness,respectively.

The cured rubber in accordance with the present invention bears ahardness of not more than 30°, preferably not more than 20° in Ahardness to not less than 15°, preferably not less than 30° in Fhardness and an impact resilience of not less than 40%, preferably notless than 50%, most preferably from 60 to 95%. Such a cured rubber is anovel rubber material which has not been known heretofore. That is, inthe case of conventional cured rubbers, those with a hardness of notmore than about 30° have an impact resilience of less than about 40%,and those with an impact resilience of not less than about 40% have ahardness of more than about 30°. It is common knowledge that the impactresilience of cured rubber declines with a decrease in its hardness. Thephysical properties of the cured rubber of the present invention, i.e.an A hardness of not more than 30°, preferably not more than 20° , andan impact resilience of not less than 40%, preferably not less than 50%,most preferably from 60 to 95%, are definitely outside of the priorcommon concept.

The rubber sheet for turntable made of the above-described unique curedrubber in accordance with the present invention exhibits such aremarkable effect that it shows an excellent vibration-absorbingproperty expecially in superlow frequency range and prevents the tonequality of reproduced sounds from deteriorating due to howling orexternal vibrations. That is, the rubber sheet effectively absorbs andshields off vibrations of the above-mentioned superlow frequencies,particularly those of not more than 10 Hz, and isolates a disc recordfrom these vibrations, so that the occurrence of howling is preventedeffectively and there are no dangers such as the modulation of pickupoutputs by these vibrations. Accordingly, especially the maximum treblesounds in soprano of female vocal sounds can be reproduced as they areand there can be also obtained excellent reproduced sounds in bassrange. Thus listeners can enjoy a listening of a record with enrichedimpressions of presence.

It is necessary that the cured rubber in accordance with the presentinvention has a hardness in the range from an A hardness of not morethan 30°, preferably not more than 20° to an F hardness of not less than15°, preferably not less than 30° and an impact resilience of not lessthan 40%, preferably not less than 50%, most preferably from 60 to 95%.Only when the hardness and impact resilience fall within the abovedefined ranges, the cured rubber can exhibit excellentvibration-absorbing properties, especially in the superlow frequencyrange and prevent the tone quality of reproduced sounds fromdeteriorating due to howling or external vibrations with maintaining thegeneral properties as a rubber sheet for turntable. If the hardness ishigher than the above range and the impact resilience is lower than theabove range, the vibration-absorbing properties become poor especiallyin the superlow frequency range. Furthermore the property being soft andresilient like baby skin, which is one of the most significant featuresof the cured rubber of the present invention, becomes lost. As a result,the rubber sheet does not come tightly in contact to the disk record orturntable, nor does it ensure a breathing phenomenon thereabove andbelow (this will be described in detail later). If both hardness andimpact resilience are below the above ranges, the rubber sheet has onlya scarce self-supporting property and cannot undergo the weights of adisc record, a cartridge and a tone arm, resulting in that its originalshape cannot be maintained. More particularly, the projections which areprovided on the rubber sheet as described later become deformed andflat, so that the contact surface area between the rubber sheet and adisc record or turntable is increased and the reproduced sounds tend tobecome heavy. Therefore, it is impossible to reproduce clear and steadysource sounds in both treble and bass regions.

The rubber sheet of the present invention can be classified into thefollowing three categories, depending on its physical properties, mainlyon the basis of differences in hardness:

(I) The rubber sheet having a hardness of from more than 15° to not morethan 30° in A hardness and an impact resilience of not less than 50%,particularly from 65 to 95%. The rubber sheet of this category permitsespecially the reproduction of sounding sounds. Consequently, forexample, sounding sounds of a pipe organ or the like can be reproducedas they are and the maximum treble sounds in the soprano of female vocalsound can be reproduced beautifully.

(II) The rubber sheet having a hardness of from 1° to 15°, preferablyfrom 2° to 7° in A hardness and an impact resilience of from 40 to 90%,more preferably from 50 to 95%, most preferably from 60 to 95%. Therubber sheet of this category possesses the advantages of both of therubber sheets belonging to the category (I) and the category (III)discussed below.

(II) The rubber sheet having a hardness from less than 1° in A hardnessto not less than 15°, preferably not less than 30° in F hardness and animpact resilience of not less than 40%, more particularly 50 to 95%. Therubber sheet of this category inhibits howling more sufficiently andpermits reproducing more excellent sounds of superlow frequencies.

It is further desirable that, in addition to the above defined physicalproperties, the cured rubber in accordance with the present inventionbears a tensile strength of 0.1 to 100 kg./cm.², more preferably 1 to 50kg./cm.², an elongation of 50 to 1,000%, more preferably 200 to 1,000%and a specific gravity of 0.8 to 1.3, more preferably 0.89 to 1.05. Ifthe tensile strength is less than the above range, the rubber sheetshows a scarce self-supporting property and becomes fragile and easilybroken upon application of small tensile force. Contrarily, if it isgreater than the above range, the rubber sheet loses flexibility andfails to come tightly in contact to the disk record or the turntable. Ifthe elongation is less than the above range, the breathing as statedabove is difficult. On the other hand, if the elongation is less thanthe above range, the rubber sheet tends to be extended to the maximumand sagged. If the specific gravity is more than the above range, anexcess load is applied to a driving motor for the turntable, providedthat the average thickness of the rubber sheet is more than 2 mm., thusamplifying vibration sounds of the motor. On the other hand, if thespecific gravity is less than the above range, it becomes impossible toadd fillers to the rubber composition, so that it is difficult to obtaina rubber sheet having a minimum self-supporting property.

Preferred physical constants for rubber sheets of the above-definedcategories (I) to (III) are as follows:

Category (I)

Tensile strength: 1 to 100 kg./cm.², particularly 5 to 50 kg./cm.²

Elongation: 100 to 1,000%, particularly 200 to 1,000%

Specific gravity: 0.89 to 1.05

Category (II)

Tensile strength: 1 to 50 kg./cm.², particularly 3 to 50 kg./cm.²

Elongation: 100 to 1,000%, more preferably 120 to 1,000%, and mostpreferably 200 to 1,000%

Specific gravity: 0.89 to 1.05, particularly 0.93 to 1.05

Category (III)

Tensile strength: 0.1 to 50 kg./cm.², particularly 0.1 to 20 kg./cm.²

Elongation: 100 to 1,000%, particularly 200 to 1,000%

Specific gravity: 0.89 to 1.05

The cured rubber having the above specific physical constants can beobtained by curing a rubber composition which comprises (A) 100 parts(parts by weight, the same hereinafter) of a rubber component, (B) 1 to2,000 parts, preferably 5 to 2,000 parts, most preferably 100 to 1,500parts of a factice and (C) 20 to 2,000 parts, preferably 50 to 2,000parts, most preferably 200 to 1,500 parts of a softening agent. Therubber composition obtained by adding such large amounts of a facticeand a softening agent to a rubber component is a new rubber composition.Only by curing such a new rubber composition, there can be obtained aparticular cured rubber having a hardness of not more than 30°,particularly not more than 20° in A hardness and an impact resilience ofnot less than 40%, preferably not less than 50%, most preferably from 60to 95%.

The rubber component (A) is not particularly limited. There areexemplified rubber components composed of predominantly one or moremembers of polynorbornene, natural rubber, isoprene rubber, chloroprenerubber, styrene-butadiene rubber, butadiene rubber, butyl rubber,ethylene-propylene rubber, ethylene-propylene-diene rubber, nitrilerubber, acryl rubber, urethane rubber, chlorinated polyethylene,chlorosulfonated polyethylene, epichlorohydrin rubber, polysulfiderubber, silicone rubber and the like. These rubber components includeregenerated rubbers thereof (for example, rubber powder or the like).Furthermore, these rubber components may be either in the form of solid(powder, pellet, block, sheet, etc.) or in the form of liquid (liquidrubber, latex, etc.). Out of these components, a rubber componentcomposed of polynorbornene as a predominant component is particularlypreferable in the present invention. In that case, it is desirable thatthe proportion of polynorbornene is not less than 50% (% by weight, thesame hereinafter), more preferably not less than 65% on the basis of thetotal weight of the rubber component. As polynorbornene, there arepreferably used those having an average molecular weight of about 10⁶ toabout 5×10⁶, more preferably about 2×10⁶ to about 3×10⁶.

The factice as the component (B) includes any of various kinds offactices such as white factice, black factice, amber factice and bluefactice which are available by vulcanizing a variety of vegetable oilsincluding linseed oil, rapeseed oil, soybean oil, sesame oil, tung oiland castor oil with use of sulfur or sulfur chloride. Those factices maybe used alone or in admixture. The factice prepared by vulcanizingrapeseed oil is most favorable.

The softening agent as the component (C) includes oils, plasticizers andother agents having a softening activity. The oils include any ofaromatic oils, naphthenic oils, paraffinic oils, vegetable oils andanimal oils and the like which are commonly used as a filling oil(softening oil, process oil, etc.) for rubbers. The vegetable oils andanimal oils mentioned above include, for example, castor oil, rapeseedoil, linseed oil, whale oil, fish oil and the like. As the plasticizers,there can be used any of those having a high softening performance outof plasticizers usually used as a plasticizer for rubbers. For example,there are exemplified dibutyl phthalate, dioctyl phthalate and dioctylsebacate. Other softening agents include liquid rubbers. The softeningagents mentioned above may be used alone or as a mixture. Usually oilalone or in mixture with a plasticizer is preferable.

Moreover, to the above-mentioned rubber composition there may be addedproperly common rubber additives including fillers such as carbon blackand zinc oxide, colorants, lubricants such as stearic acid andantioxidants to the extent that they do not impair the above-discussedphysical properties, in addition to the components (A) to (C).

Preferred rubber compositions for the rubber sheets of the categories(I) to (III) are as follows:

Category (I)

Component (A) 100 parts

Component (B) 1 to 1,500 parts, preferably 5 to 1,000 parts and mostpreferably 100 to 1,000 parts

Component (C) 20 to 1,500 parts, more particularly 200 to 1,000 parts

Category (II)

Component (A) 100 parts

Component (B) 5 to 1,500 parts, preferably 5 to 500 parts and mostpreferably 100 to 500 parts

Component (C) 50 to 1,500 parts, more particularly 200 to 1,000 parts

Category (III)

Component (A) 100 parts

Component (B) 20 to 2,000 parts, preferably 50 to 1,500 parts and mostpreferably 100 to 1,500 parts

Component (C) 50 to 2,000 parts, preferably 200 to 1,500 parts

For curing of the above rubber composition, any one of usual curingsystems can be used. Either sulfur curing or sulfurless curing may beused. The curing conditions, etc. are not particularly limited andnormal conditions are adopted.

A typical example of the above rubber composition, in the case of sulfurcuring, is as follows:

    ______________________________________                                        (Component)      (Parts)                                                      ______________________________________                                        Rubber component 100                                                          Factice          100 to 1,500                                                 Softening agent  200 to 1,500                                                 Filler            1 to 200                                                    Antioxidant      0.5 to 6                                                     Sulfur           0.5 to 10                                                    Curing accelerator                                                                             1 to 20                                                      ______________________________________                                    

As another feature of the rubber sheet for turntable in accordance withthe present invention, there is no need for after-treatments which aregenerally adopted in the case of a conventional rubber sheet forturntable, for example, the application of halogenation treatment forensuring increased surface hardness to make the rubber sheet notsusceptible to flaws and the application of a coating onto the surfacethereof for preventing deposit of dust. While being very low in hardnessas set forth above, the rubber sheet for turntable in accordance withthe present invention is high in impact resilience. For the reason, therubber sheet has characteristic properties that it is hardly susceptibleto flaws and that dust is hardly deposited thereon. Moreover, the rubbersheet has a very convenient characteristic property that even when flawis made or dust is attached on the rubber sheet, the flaw disappears andthe dust is removed, if the rubber sheet is swung or crumpled with easelike cloth since it is very soft. The rubber sheet of the presentinvention has another characteristic property, not experienced with aconventional rubber sheet, that washing in water is effective and inthat case the rubber sheet can be cleaned by washing with crumpling.

The rubber sheet for turntable in accordance with the present inventionis subjected to no particular limitations as to configuration and any ofconventional configurations can be adopted. However, by limiting theconfiguration of the rubber sheet to the following particular ones, moreexcellent vibration-aborbing effect and tone quality improving effectcan be exhibited in combination with the above-mentioned specificphysical properties.

That is, a preferable configuration of the rubber sheet for turntable inaccordance with the present invention includes one wherein a largenumber of projections are provided on one surface of its disk-shapedsheet body.

Since the rubber sheet with the above configuration contacts a turntableor a disc record only atop the projections thereof, the contact area ismarkedly small and therefore external vibrations are hardly transmittedto the disc record. Furthermore, the turntable rubber sheet with theabove configuration exhibits such a unique effect as in the followings:

That is, the substance with the smallest velocity of sound is rubber andthat with the second smallest velocity of sound is air. When the surfaceof the rubber sheet with the above specific configuration is in contactwith the turntable, a thin layer of air is present between the rubbersheet and the turntable by means of the projections so that the air inthis layer moves inwardly and outwardly through respective ones of theprojection-to-projection spaces during the revolution of the turntable,as if the human being breathes. Consequently, the rubber sheet and thedisc record thereon would be in such a situation as resting on air. Thisinsures complete isolation of the external vibrations and an improvementin tone quality. Similarly, in the case where the surface of the rubbersheet with the above specific configuration is in contact with the discrecord, the foregoing specific situation occurs between the rubber sheetand the disc record. Thus, the above configuration effectively shieldsoff vibrations of superlow frequencies, particularly not more than 10 Hzand prevents these vibrations from being transmitted to the disc record.For this reason, the occurrence of howling is effectively prevented andthere is no danger that pickup outputs would be modulated with thesevibrations.

In the rubber sheet with the above specific configuration, the shape ofthe projections is not particularly limited and various shapes can beadopted. However, in view of reducing the contact area with theturntable or the disc record as small as possible, such a shape that thearea of the head of each projection is as small as possible ispreferable. Such a projection includes one having a shape wherein atleast the head of the projection is ridge-shaped to make substantially aline contact with the turntable or the disc record. The shapes accordingto this embodiment may be, for example, roof-shaped and semicircular insection. A projection with knife edge-shaped head is particularlypreferred. Another desirable projection includes one having a shapewherein at least the head of the projection is pointed to makesubstantially a point contact with the turntable or the disk record. Theshapes according to this embodiment include a pyramid (including apolygonal pyramid, e.g. a triangular pyramid, a quadrangular pyramid andso on, the same hereinafter), a cone (including an elliptical cone, thesame hereinafter), a dome (including a near sphere, a hemisphere, acolumn with a rounded upper portion and the like, the same hereinafter)and the like. A projection having such a sharp head as a needle isparticularly preferred. For the projections with the two different kindsof shape, it is enough that at least the head thereof is ridge-shaped orpointed, and the shape of the root thereof is not particularly limited.For example, such a projection may have a shape wherein a head having aroof shape, semicircular shape, a pyramid, a cone, a dome or the like isrode on a root of a head-free pyramid, a head-free cone or the like.Furthermore, other shapes are available as long as the contact area withthe turntable or the disc record is held as small as possible. Forexample, head-free pyramidal, pillar-shaped or column-shaped projectionsmay be used. These projections may be provided at their top withrecesses to ensure a line contact or a point contact.

It is preferable that the contact area between the projection-bearingsurface and the turntable or the disk record be 0.01 to 10%,particularly 0.01 to 1% of the entire area of one surface of the rubbersheet (i.e. the area of a circle having the same diameter as that of therubber sheet, the same hereinafter).

The height of the projections is preferably 0.1 to 5 mm., moreparticularly 0.3 to 1.8 mm. When the height is outside the above range,the air-breathing behavior as mentioned above occurs with difficulty.

The projections may be disposed regularly or at random on the body ofthe rubber sheet. Regular alignments may be concentrically circular,spiral, radial, etc. The individual projections may be either spaced atan appropriate interval or closely aligned in contact with one another,or the combination of these alignments may be also adopted. When theseprojections are closely aligned, the adjacent projections may beconnected integrally with each other at their roots (in the case wherethe projections are closely aligned in concentrically circular arrays insuch a manner, its configuration is similar to a configuration whereinconcentrically circular ribs are provided on the body of the rubbersheet, each ribs being provided with closely spaced recesses).Preferable alignment includes one wherein the projections are closelyaligned in concentrically circular arrays or the like so that air isheld in a space between the rubber sheet and the turntable or the discrecord by the arrays of the projections. According to the arrangement ofthis embodiment, the air involved passes through the gaps between theclosely aligned projections, thus providing better breathing behavior.As a result, more excellent vibration-damping effect and tone qualityimproving effect can be exhibited.

The projections as mentioned above may be formed integrally with therubber sheet, or formed separately from the rubber sheet and affixedthereto by means such as bonding.

Although the rubber sheet of the above specific configuration has theprojections on the one surface, the other surface of the rubber sheet isfree of particular limitaitons as to configuration. For example, theother surface may be completely flat or provided with the sameprojections as set forth above or with concentrically circular, spiralor radial grooves or ribs. Furthermore, recesses or projections eachhaving a circular shape, a polygonal shape in plan view may be alignedregularly (e.g., concentrically circularly, spirally or radially) or atrandom.

Preferred configurations of the rubber sheet for turntable in accordancewith the present invention will be explained by reference to thedrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are, respectively, partial plan views of respectivesurfaces of a rubber sheet for turntable in accordance with anembodiment of the present invention.

FIG. 3 is a vertical cross-sectional view taken on the line (V)--(V) inFIGS. 1 and 2.

FIG. 4 is an enlarged cross-sectional view taken on the line (W)--(W) inFIG. 1.

FIG. 5 is a vertical cross-sectional view showing an example of theshape of the edge portion of the rubber sheet for turntable inaccordance with the present invention (i.e. a vertical cross-sectionalview taken on the line (X)--(X) in FIG. 1).

FIG. 6 is a partial perspective view of an example of an array ofprojections.

FIGS. 7 and 8 are vertical cross-sectional views showing an example of aprojection with recesses.

FIG. 9 is a partial perspective view of an example of an array whereprojections of different shapes are used in combination.

FIG. 10 is a partial, vertical cross-sectional view showing a situationwhere the rubber sheet for turntable in accordance with the presentinvention is mounted on a turntable and a disc record is mounted on therubber sheet.

FIGS. 11 to 13 are, respectively, plan views of the rubber sheet forturntable in accordance with the present invention, wherein a pluralityof insular arrays of projections are provided.

FIGS. 14 and 15 are, respectively, plan views of respective surfaces ofa rubber sheet for turntable in accordance with another embodiment ofthe present invention.

FIG. 16 is a vertical cross-sectional view taken on the line (Y)--(Y) inFIGS. 14 and 15.

FIGS. 17 and 18 are, respectively, plan views of respective surfaces ofa rubber sheet in accordance with further another embodiment of thepresent invention.

FIG. 19 is a vertical cross-sectional view taken on the line (Z)--(Z) inFIGS. 17 and 18.

FIGS. 20a and 20b, 21a and 21b, 22a and 22b and 23a and 23b are topviews and cross-sectional views of the conventional rubber sheets forturntable, respectively.

Referring to FIGS. 1 to 4, (1) is the body of a rubber sheet and (2) isa center shaft hole. One surface of the body (1) is provided withprojections (3) aligned closely in contact with one another inconcentrically circular arrays (cf. FIGS. 1, 3 and 4) (this arrangementof projections is called "Embodiment A" hereinafter). In the followings,such a concentrically circular array of projections (3) wherein theprojections (3) are aligned closely will be referred to simply as "array(4) of projections". The number of the arrays (4) of projections is notparticularly limited as long as the body (1) of the rubber sheet is heldhorizontally on the turntable and may be usually about 3 to about 10.The arrays (4) of projections may be substantially equally spaced alongthe radial direction of the body (1) of the rubber sheet, or asindicated in FIGS. 1 and 3, two or more arrays (4) may be disposedclosely with one another into a block [for example, the two arrays (4b)and (4c), (4d) and (4e), and (4f) and (4g )], a plurarity of such blocksbeing disposed on the body (1). The arrays (4) of projections may bedisposed on concentrically circularly aligned ribs (5) as shown in FIG.3.

Although in Embodiment A the projections (3) in respective arrays (4)are usually flush at their tops, they should not be limited thereto. Forexample, when the surface bearing the projections (3) is to be incontact with the disk record, there is given an embodiment where theprojections (3) are highest in the most inner array (4a) (the array isprovided just outside the label section of the disc record) and lowergradually in the order of the arrays (4b), (4c), (4d), (4e), (4f) and(4g) so that the disc record is substantially held by means of the mostinner array (4a) of projections and in no direct contact with theremaining arrays (4b), (4c), (4d), (4e), (4f) and (4g) of projectionsother than the array (4a). In this embodiment, the disc record ismaintained under non-contacting relationship with the rubber sheet onits periphery when the disk record is stationary. However, when acartridge is in operating position for playback of the disc record, thedisk record comes into contact with the rubber sheet due to the weightof the cartridge so that the contact area with the rubber sheet issmall. This insures a further improvement in isolation of externalvibrations.

The most outer array (4g) is usually disposed on an edge portion of thebody (1) of the rubber sheet. It is preferable that, as illustrated in avertical cross-sectional view of FIG. 5 (corresponding to an enlargedcross-sectional view taken on the line (X)--(X) in FIG. 1), the edgeportion of the body (1) is outwardly inclined and the projections of themost outer array (4g) are disposed on this inclined edge portion.According to this embodiment, the tension resistance of the rubber sheetis reduced in the edge portion of the body (1) due to its thin thicknessand the tight contact of the rubber sheet with the turntable is furtherimproved.

In Embodiment A, the projections (3) may take the various shapes asdescribed previously. Preferable shape is one where the head of theprojection (3) is ridge-shaped. FIG. 4 depicts an example of suchprojections (3) which are of mountain profile with a downwardlyexpanding skirt. For the projections (3) with the ridge-shaped heads, itis preferable that the ridges (3a) thereof extend along the radialdirection of the body (1) of the rubber sheet as shown in FIG. 1. Inthat case, with respect to two adjacent arrays [e.g., (4d) and (4e)],the projections (3) in the array (4d) may overlap with those in theother array (4e) along the radial direction of the body (1) of therubber sheet. However, it is desirable that the ridges (3a) of theprojections (3) in the array (4d) are located in agreement with valleys(6) between two adjacent projections (3) in the other array (4e) toavoid such overlapping relationship as seen from FIG. 4. According tothis construction, the above-mentioned breathing behavior in a layer ofair confined between the rubber sheet and the turntable or the discrecord is effected more favorably, thus shielding off the externalvibrations more sufficiently.

Another preferred shape of the projection (3) in Embodiment A is onewhere the head of the projection (3) is pointed. A partial perspectiveview of FIG. 6 [wherein the concentrically circular array (4) ofprojections is re-aligned in a straight line] depicts an example of sucha shape of the projections (3). The projection (3) shown in FIG. 6 is aquandrangular pyramid. In the case of the projections (3) with thepointed heads, with respect to the projections (3) of two adjacentarrays [e.g., (4d) and (4e)], the projections (3) in the array (4d) mayoverlap with those in the other array (4e) along the radial direction ofthe body (1) of the rubber sheet but the pointed heads (3d) of theprojections (3) in the array (4d) are preferably located in agreementwith the valley (6) between two adjacent projections (3) in the otherarray (4e) along the radial direction of the body (1) of the rubbersheet to avoid such overlapping relationship, in the same manner as inthe case of the projections (3) with the ridge-shaped heads. Each of theforegoing projections (3) has a single summit (3b) but may have two ormore summits (3a') which may be formed by providing the ridge of theabove-mentioned projection (3) having the ridge-shaped head withrecesses as shown in FIGS. 7 and 8. FIG. 7 is a vertical cross-sectionalview showing one of the projections (3) with the ridge-shaped head alongthe radial direction of the body (1) of the rubber sheet and FIG. 8 is avertical cross-sectional view along the circumferential direction.

Furthermore, in Embodiment A, the projections (3) with the ridge-shapedheads and those with the pointed heads may be used in combination orthese projections (3) may be used in combination with projections (3) ofother different shapes in the particular array (4). An example of suchcombination is illustrated in a perspective view of FIG. 9 [but aperspective view wherein the concentrically circular array (4) ofprojections is realigned in a straight line]. In FIG. 9, the projections(3) with the pointed heads and the projections (3) with the ridge-shapedheads [but the latter is different from that in FIG. 4 in that theridges (3a) are aligned along the circumferential direction of the body(1) of the rubber sheet] are aligned alternatively with each other.

As illustrated in FIGS. 2 and 3, it is desirable that the oppositesurface of the rubber sheet provided with the projections (3) inaccordance with Embodiment A is provided with concentrically circulargrooves (7) in position to correspond to the respective arrays (4) ofprojections so that the rubber sheet is substantially uniform inthickness as a whole. In that case, each of the grooves (7) may beformed in correspondence with one of the individual arrays (4) of theprojections or in correspondence with a plurality of adjacent arrays (4)of projections as shown in FIG. 3 [for example, one of the grooves (7)is provided for each of the ribs (5)]. With such a configuration, theweight of the rubber sheet is reduced and a layer of air is confined inthe grooves (7), thus providing an improvement in vibration absorptionand tone quality. In the case of the rubber sheet having the arrays (4)of projections on the one surface and the grooves (7) on the oppositesurface, the tone quality of reproduced sounds differs delicatelydepending on which surface of the rubber sheet is to be in contact withthe turntable (or the disc record). Therefore it is provided anadvantage that proper selection of the surface of the rubber sheet to bein contact with the turntable (or the disc record) results in providingagreeable reproduced sounds.

The turntable is usually provided with openings in the intermediateportion between the center shaft and the periphery thereof. It is thusfavorable that, when the surface provided with the grooves (7) is to bein contact with the turntable, the most inner groove (7) be kept awayfrom the openings and a layer of air be confined between at least themost inner groove (7) and the turntable.

In the case where the surface provided with the arrays (4) ofprojections is in contact with the disc record and the other surfaceprovided with the grooves (7) in contact with the turntable inaccordance with the present invention, the preferred configuration ofthe rubber sheet is that wherein the projections (3) are highest in themost inner array (4a) and lower gradually in the order of the arrays(4b), (4c), (4d), (4e), (4f) and (4g) and at least the most inner groove(7) is kept away from the openings of the turntable. FIG. 10 depicts asituation where a rubber sheet of such a configuration is mounted on theturntable and the disc record is rested on the rubber sheet. In FIG. 10,(8) is the turntable, (9) is the center shaft, (10) is the openings ofthe turntable and (11) is the disc record. In the situation shown inFIG. 10, the rubber sheet is in tight contact at its central portion(corresponding to the label region of the disc record) with theturntable (8). Over the central portion of the rubber sheet which is intight contact with the turntable, there is involved a layer of air (12)between the disc record (11) and the most inner array (4a) of theprojections on the rubber sheet. The air in the layer (12) passesinwardly and outwardly through the gaps between the adjacent projectionsin the array (4a) to thereby perform the breathing behavior as statedabove during reproduction of the disc record. On the other hand, themost inner groove (7a) of the rubber sheet is kept away from theopenings (10) of the turntable (8) so that a layer of air ishermetically sealed within the groove (7a). This combination of thecentral portion of the rubber sheet which is in tight contact with theturntable, the layer (12) of air performing the breathing behavior andthe layer of the hermetically sealed air in the groove (7) exhibitsparticularly excellent vibration-damping effect and tone qualityimproving effect.

While in the foregoing there is explained a configuration of the rubbersheet wherein air is confined between the rubber sheet and the discrecord or the turntable by the concentrically circular arrays (4) of theclosely aligned projections, the arrangement of the projectionseffective to confine air is not limited thereto. For example, asillustrated in plan views of FIGS. 11 to 13, there are arrangementswherein a plurality of circular (including elliptic) arrays of closelyaligned projections (3), a plurality of polygonal (including a triangle,a quadrangle and so on) arrays of closely aligned projections (3), aplurality of fan-shaped arrays of closely aligned projections (3), etc.(those will be referred to as "insular arrays (13) or projections"hereinafter) are disposed outside the label section of the disk record.According to this embodiment, air is confined by means of the individualinsular arrays (13) or projections to provide breathing behavior. Theinsular arrays (13) of projections may not be of the same dimention andfor example smaller insular arrays (13) may be interposed among greaterinsular arrays (13) as shown in FIG. 11. Moreover, the differentlyshaped insular arrays (13) of projections may be used together as shownin FIG. 13. In this embodiment, the concentrically circular array (4) ofprojections may be disposed at the central portion or peripheral portionof the body (1) of the rubber sheet as shown in FIGS. 11 and 12.

FIG. 14 is a plan view of one surface of the rubber sheet for turntablein accordance with another embodiment of the present invention, FIG. 15is a plan view of the other surface of the rubber sheet and FIG. 16 is avertical cross-sectional view taken on the line (Y)--(Y) in FIGS. 14 and15. In the one surface of the body (1) of the rubber sheet there areprovided projections (3) aligned closely with one another in a spiralarray (see FIG. 14) (this arrangement of projections is referred to as"Embodiment B" hereinafter). In the following, such a spiral array ofthe closely aligned projections (3) will be simply named "array (14) ofprojections".

In Embodiment B, the respective projections (3) in the array (14) may beall flush at their tops or lower gradually in a direction toward theperiphery of the rubber sheet for the same reason as in Embodiment A.Each projection (3) may also take the various shapes as described abovebut a projection having a ridge-shaped head or a pointed head ispreferable as in Embodiment A. In the case of the projections having theridge-shaped head, it is preferable that the ridges be alignedsubstantially along the radial direction of the body (1) of the rubbersheet as shown in FIG. 14. The projections with the ridge-shaped headmay be provided with recesses to form two or more summits as shown inFIGS. 7 and 8. Furthermore, in the array (14) of projections, theprojections with the ridge-shaped head and the projections with thepointed head may be used in combination and further those projectionsmay be used in combination with projections of other shapes as shown inFIG. 9. In Embodiment B, preferably the edge portion of the body (1) ofthe rubber sheet is outwardly inclined in a way similar to Embodiment A.

With regard to the other surface of the rubber sheet provided with theprojections (3) according to Embodiment B, it is desirable that aportion thereof corresponding to the array (14) of projections is madeconcave as shown in FIGS. 15 and 16 [as a result, a spiral convex rib(15) is formed in the remaining portion] to thereby make the thicknessof the rubber sheet substantially uniform as a whole. The edge portionof the other surface is usually provided with a flange portion (16) toconfine air.

FIG. 17 is a plan view of one surface of the rubber sheet for turntablein accordance with further another embodiment of the present invention,FIG. 18 is a plan view of the other surface of the rubber sheet and FIG.19 is a vertical cross-sectional view on the line (Z)--(Z) in FIGS. 17and 18. On the one surface of the body (1) of the rubber sheet there areprovided projections (3) aligned closely with one another in radialarrays (see FIG. 17) (this arrangement of the projections is referred toas "Embodiment C" hereinafter). In the following, the radial arrays ofthe closely aligned projections will be simply named "array (17) ofprojections".

In Embodiment C, the respective projections (3) in the arrays (17) maybe all flush at their tops or lower gradually in a direction toward theperiphery of the rubber sheet for the same reason as in Embodiment A.Each projection (3) may take the various shapes as described above, buta projection (3) having a ridge-shaped head or a pointed head ispreferable as in Embodiment A. The projections with the ridge-shapedhead may be provided with recesses to form two or more summits as shownin FIGS. 7 and 8. In the array (17) of projections, the projections withthe ridge-shaped head and the projections with the pointed head may beused in combination and further these projections may be used incombination with projections of other shapes as shown in FIG. 9. InEmbodiment C, preferably the edge portion of the body (1) of the rubbersheet is outwardly inclined as in Embodiment A.

With regard to the other surface of the rubber sheet provided with theprojections (3) according to Embodiment C, it is also desirable thatportions thereof corresponding to the arrays (17) of projections aremade radially concave to form radial concave parts (18) in order to makethe thickness of the rubber sheet substantially uniform as a whole, asshown in FIGS. 18 and 19.

In Embodiments A to C, the projections (3) are aligned closely with oneanother but the arrangement of the projections (3) is not necessarilylimited thereto. They may be aligned at any appropriate interval.

The rubber sheet for turntable in accordance with the present inventionis free of any particular limitation as to thickness. However, therubber sheet usually has a maximum thickness in the order of 2 to 8 mm.,a minimum thickness in the order of 0.1 to 3 mm. and an averagethickness in the order of 0.1 to 6 mm., more particularly 0.5 to 2 mm.

The rubber sheet of the present invention may be made in a conventionalmanner. For example, the above specified rubber composition is directlypress-cured in a mold of a given configuration. As an alternative, anuncured composition in the form of sheet is prepared by calender moldingor extrusion molding and then subjected to press curing. Injectionmolding is also available. It is desirable that the projections (3) areformed in the mold at one time, but they may be shaped by cutting aftercuring. Furthermore, the projections (3) may be made independently ofthe body (1) of the rubber sheet and bonded to the body (1) of therubber sheet with an appropriate adhesive or the like.

BEST MODE FOR CARRYING OUT THE INVENTION

The rubber sheet for turntable in accordance with the present inventionwill be explained by referring to Examples and Comparative Examples.

EXAMPLES 1 TO 4

Using the rubber compositions as shown in Table 1, rubber sheets forturntable were manufactured.

                  TABLE 1                                                         ______________________________________                                        Rubber Composition (in parts)                                                            Examples                                                                      1       2       3         4                                        ______________________________________                                        Norsorex 150                                                                  NA (Note 1)  250       250     250     250                                    White factice S                                                               (Note 2)     225       20      260     120                                    Sunthene 255 (Note 3)                                                                      410       200     690     280                                    Dioctyl phthalate                                                                          45        10      --      --                                     FEF carbon   50        50      25      50                                     Zinc oxide   5         5       5       5                                      Stearic acid 1         1       1       1                                      Sumilizer MDP                                                                 (Note 4)     2         2       2       2                                      Sulfur       2.5       2       2       2                                      Sunceller CZ                                                                  (Note 5)     10        8       12      9                                      ______________________________________                                         (Note 1): Polynorbornene (Norsorex (registered trademark), average            molecular weight of not less than 2 × 10.sup.6) extended with 150       parts of naphthenic oil on the basis of 100 parts of polynorbornene, sold     by Nippon Zeon Co., Ltd.                                                      (Note 2): White factice made by Temma Sabu Kako Kabushiki Kaisha              (Note 3): Naphthenic oil made by Japan Sunoil Co., Ltd.                       (Note 4): 2,2'-Methylenebis(4-methyl-6-tert. butylphenol) made by Sumitom     Chemical Co., Ltd.                                                            (Note 5): N--cyclohexyl2-benzothiazyl sulfenamide made by Sanshin Kagaku      Kabushiki Kaisha.                                                        

The rubber component was first scoured at about 60° C. and kneaded withadditional components by means of a Banbury mixer and further a rollerand sheeted out to give a sheet of about 2 mm. to about 3 mm. thick. Adisc was cut out from the sheet and cured by a press machine with agiven mold under a pressure of 150 kg./cm.² and at a temperature of 155°C. for 20 min.

Thus there were obtained rubber sheets for turntable having theconfiguration as shown in FIGS. 1 to 4 and 10; a diameter of 292 mm; amaximum thickness of 3.7 mm; a minimum thickness of 1.9 mm; a height ofthe projections (3) in the most inner array (4a) of 0.5 mm. [the height(H₁) in FIG. 3] or 1.5 mm. [the height (H₂) in FIG. 3]; a length of theridges of the projections (3) of about 3 mm. to about 6.5 mm; a spacingbetween adjacent projections (3) of about 5 mm. to about 6 mm; a depthof the grooves (7) of 1 mm; and a width of the grooves (7) (in theradial direction) of about 30 mm. (the individual projections (3) weremade gradually lower in height toward the periphery of the rubbersheet).

With respect to the obtained rubber sheets, various physical propertieswere measured. The results are shown in Table 2. Measurements of impactresilience, tensile strength and elongation indicated in Table 2 wereconducted according to JIS K 6301-1969.

                  TABLE 2                                                         ______________________________________                                                       Examples                                                       Physical constants                                                                             1       2       3     4                                      ______________________________________                                        A hardness (degree)                                                                            4       5       7     9                                      Impact resilience (%)                                                                          68      60      72    74                                     Tensile strength (kg./cm..sup.2)                                                               8       21      19    10                                     Elongation (%)   398     575     367   281                                    Specific gravity 0.977   0.982   1.003 0.987                                  ______________________________________                                    

Total area of the ridges of the projections (3) (852 in total number) ineach rubber sheet of Examples 1 to 4 occupied about 1.45% of the entirearea of the one surface of the rubber sheet (assuming that the width ofthe ridges was 0.25 mm.). When a glass sheet having the same dimensionas that of the rubber sheet and a thickness of 2 mm. (weight: about 300g.) was mounted on the one surface of the rubber sheet bearing theprojections (3), the glass sheet was substantially in contact with therubber sheet only at the ridges of the projections (3) in the most innerarray (4a). In that case, the ratio of the contact area to the entirearea of the one surface of the rubber sheet was about 0.1%. Thepreferred configuration of the rubber sheet for turntable in accordancewith the present invention is characterized in that the contact area issmall in such a way.

Furthermore, the following tests were carried out on the rubber sheetsof Examples 1 to 4. For comparison, the same tests were also carried outon four sorts of rubber sheets attached to players commerciallyavailable on the market. The rubber sheets used herein are summarized inTable 3.

                  TABLE 3                                                         ______________________________________                                        Comparative                                                                   Examples     1        2        3      4                                       ______________________________________                                        Material     Butyl    SBR/NR   Styrene-                                                                             Butyl                                                rubber   (Note 1) butadiene                                                                            rubber                                                                 rubber (Note 2)                                A hardness (degree)                                                                        24       50       57     36                                      Impact resilience (%)                                                                      19       --       33     --                                      Specific gravity                                                                           1.223    1.356    1.431  1.642                                   Halogenation No       Yes      Yes    Yes                                     Configuration                                                                              FIG. 20a FIG. 21a FIG. 22a                                                                             FIG. 23a                                (Note 3)     FIG. 20b FIG. 21b FIG. 22b                                                                             FIG. 23b                                ______________________________________                                         (Note 1): Mixture of styrenebutadiene rubber and natural rubber               (Note 2): The body made of butyl rubber is covered with a mixture of          natural rubber and styrenebutadiene rubber.                                   (Note 3): FIGS. 20a and 20b, 21a and 21b, 22a and 22b, and 23a and 23b ar     top and crosssectional views of the rubber sheets of Comparative Examples     1 to 4, respectively. In these drawings, thicknesses (h.sub.1), (h.sub.2)     (h.sub.3), (h.sub.4), (h.sub.5), (h.sub.6), (h.sub.7) and (h.sub.8) of th     rubber sheets are as follows: (h.sub.1): 4.2 mm. (h.sub.2): 3.5 mm.           (h.sub.3): 5 mm. (h.sub.4 ): 4 mm. (h.sub.5): 3.5 mm. (h.sub.6): 2.5 mm.      (h.sub.7): 5 mm. (h.sub.8): 4.5 mm.                                      

The summaries of configurations were as follows:

Comparative Example 1

Top surface: There is provided a circular recess of 0.7 mm. deep at itscenter.

Bottom surface: flat

Comparative Example 2

Top surface: There are provided a circular recess of 1.0 mm. deep at itscenter and a concentrically circular groove of 1.0 mm. deep.

Bottom surface: flat

Comparative Example 3

Top surface: There is provided a circular recess of 1.0 mm. deep at itscenter.

Bottom surface: flat

Comparative Example 4

Top surface: There are provided three concentrically circular grooves of0.5 mm. deep.

Bottom surface: flat

It is noted that the bottom surfaces of the rubber sheets of ComparativeExamples 1 to 4 are all flat and the flat surfaces are to be in contactwith the turntable. In the following tests, therefore, the rubber sheetsof Comparative Examples 1 to 4 were mounted so that their flat bottomsurface was in contact with the turntable.

(1) Tone quality test

(i) Play-back equipment

Pre-main amplifier: AU-D907 (made by Sansui Electric Co., Ltd.)

Player: DP-40F (made by Nippon Columbia Co., Ltd.)

Cartridge: V-15 type 3 (made by Shure Brothers, Inc.)

Loudspeaker: DS-401 (made by Mitsubishi Electric Corporation)

(ii) Location

About 40-tatami mat room (equipped with an air conditioner and aventilator)

Floor: Vinyl-tiled (with mortared underlying layer)

Ceiling: Flat noninflammable material (no sound-absorption opening)

Inner walls: Plywood

(iii) Test method

A panel of 4 trained listeners A, B, C and D was instructed to hear thefollowing records and score them on the following parameters. The rubbersheets of Examples 1 to 4 were set on the turntable so that the surfaceillustrated in FIG. 1 [the surface provided with the projections (3)]was in contact with the record.

(α) Female vocal

Title of record: Valses, Canciones Y Tangos (Nippon Phonogram Co., Ltd.)

Singer: Ginamaria Hidalgo

Accompaniment: Hugo Videla (guitar) and others

Program: Te Pido Que Lo Pienses

Parameters:

(a) if reproduced sounds are near human sounds

(b) if the treble region of soprano is not distorted

(c) if accompanying guitar sounds are not stronger than vocal sounds

(d) if the impressions of presence are rich

(β) Concerto

Title of record: "Le Quattro Stagioni" #8 by Vivaldi (Nippon PhonogramCo., Ltd.)

Performer: Roberto Michelucci (violin) and I Musici

Program: Concerto No. 1, E Major RV 269 "Spring"

Parameters:

(a) if sounds of chords are vivid

(b) if the treble region is not distorted

(c) if the bass region is well reproduced

(d) if the impressions of presence are rich

(γ) Flamenco guitar

Title of record: Manolo Sanlucar in Japan (RVC Corporation)

Performer: Manolo Sanlucar (guitar), Isidro (guitar) and Jose Miguel(piano)

Program: Callejon del Carmen

Parameters:

(a) if sounds of chords are well reproduced

(b) if the treble region is not distorted

(c) if beats of a guitar are well reproduced

(d) if the impressions of presence are rich

The scores as to the above parameters were determined according to thefollowing five-point system.

5 points: Exceptionally good

4 points: Fairly good

3 points: Good

2 points: Ordinary

1 point: Poor

The sum of the points as to each of the parameters evaluated by each ofthe listeners multiplied by five was regarded as overall evaluation byeach of the listeners (if all of the parameters are evaluated as 5points, overall evaluation would be full marks (100 points). Also, theaverage value of overall evaluations by the four listeners was computed.

The results thereof are shown in Tables 4 to 9.

                                      TABLE 4                                     __________________________________________________________________________    (Female vocal)                                                                        Example 1    Example 2  Example 3                                                                              Example 4                            Parameters                                                                            A  B  C  D  A  B  C D  A  B C D A B C D                               __________________________________________________________________________    Nearness to                                                                   human voices                                                                          5  5  5  5  5  5  5 5  5  5 5 5 5 5 5 5                               Distortion                                                                    of treble                                                                             5  5  5  5  5  5  5 5  5  5 3 5 3 5 3 5                               Accompanying                                                                  guitar  5  5  5  5  5  5  3 5  5  5 5 3 5 3 5 5                               Presence                                                                              5  5  5  5  5  5  5 5  5  3 5 5 5 3 5 5                               Overall                                                                       evaluation                                                                            100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              90                                                                              100                                                                              100                                                                              90                                                                              90                                                                              90                                                                              90                                                                              80                                                                              90                                                                              100                             Average 100         98         92       90                                    __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    (Female vocal)                                                                        Comparative                                                                           Comparative                                                                           Comparative                                                                           Comparative                                           Example 1                                                                             Example 2                                                                             Example 3                                                                             Example 4                                     Parameters                                                                            A B C D A B C D A B C D A B C D                                       __________________________________________________________________________    Nearness to                                                                   human voices                                                                          3 3 5 3 1 2 2 2 1 2 2 2 1 2 2 2                                       Distortion                                                                    of treble                                                                             1 5 3 3 1 2 2 2 1 1 1 1 1 2 2 2                                       Accompanying                                                                  guitar  2 3 3 2 2 2 2 2 1 2 2 2 1 2 2 2                                       Presence                                                                              3 3 3 3 2 2 2 2 1 2 2 2 2 2 2 2                                       Overall                                                                       evaluation                                                                            45                                                                              70                                                                              70                                                                              55                                                                              30                                                                              40                                                                              40                                                                              40                                                                              20                                                                              35                                                                              35                                                                              35                                                                              25                                                                              40                                                                              40                                                                              40                                      Average 60      38      31      36                                            __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    (Concerto)                                                                    Example 1          Example 2                                                                               Example 3                                                                             Example 4                                Parameters                                                                          A  B  C  D  A B  C D  A B C D A B C D                                   __________________________________________________________________________    Sounds of                                                                     chords                                                                              5  5  5  5  5 5  5 5  3 5 5 3 5 3 5 5                                   Distortion                                                                    of treble                                                                           5  5  5  5  3 5  3 5  5 5 5 5 3 5 3 3                                   Bass  5  5  5  5  5 5  5 5  5 3 3 5 5 5 5 5                                   Presence                                                                            5  5  5  5  5 5  5 5  5 5 5 5 3 5 5 5                                   Overall                                                                       evaluation                                                                          100                                                                              100                                                                              100                                                                              100                                                                              90                                                                              100                                                                              90                                                                              100                                                                              90                                                                              90                                                                              90                                                                              90                                                                              80                                                                              90                                                                              90                                                                              90                                  Average                                                                             100         95        90      88                                        __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    (Concerto)                                                                    Comparative   Comparative                                                                           Comparative                                                                           Comparative                                     Example 1     Example 2                                                                             Example 3                                                                             Example 4                                       Parameters                                                                          A B C D A B C D A B C D A B C D                                         __________________________________________________________________________    Sounds of                                                                     chords                                                                              3 3 3 3 3 2 3 3 2 2 2 2 3 2 3 3                                         Distortion                                                                    of treble                                                                           5 5 3 5 2 2 3 2 2 3 3 2 2 2 1 2                                         Bass  2 3 3 3 2 3 3 3 2 3 2 2 2 3 3 2                                         Presence                                                                            3 5 3 3 2 1 2 2 2 3 2 2 3 2 1 2                                         Overall                                                                       evaluation                                                                          65                                                                              80                                                                              60                                                                              70                                                                              45                                                                              40                                                                              55                                                                              50                                                                              40                                                                              55                                                                              45                                                                              40                                                                              50                                                                              45                                                                              40                                                                              45                                        Average                                                                             69      47      45      45                                              __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    (Flamenco guitar)                                                             Example 1          Example 2  Example 3                                                                             Example 4                               Parameters                                                                          A  B  C  D  A  B C  D  A B C D A B C D                                  __________________________________________________________________________    Sounds of                                                                     chords                                                                              5  5  5  5  5  3 5  5  5 5 3 3 5 3 5 3                                  Distortion                                                                    of treble                                                                           5  5  5  5  5  5 5  5  5 5 5 5 5 5 5 5                                  Beats of                                                                      guitar                                                                              5  5  5  5  5  3 5  5  3 3 5 5 3 3 5 5                                  Presence                                                                            5  5  5  5  5  5 5  5  5 5 3 5 5 5 3 5                                  Overall                                                                       evaluation                                                                          100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              80                                                                              100                                                                              100                                                                              90                                                                              90                                                                              80                                                                              90                                                                              90                                                                              80                                                                              90                                                                              90                                 Average                                                                             100         95         88      88                                       __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________    (Flamenco guitar)                                                             Comparative   Comparative                                                                           Comparative                                                                           Comparative                                     Example 1     Example 2                                                                             Example 3                                                                             Example 4                                       Parameters                                                                          A B C D A B C D A B C D A B C D                                         __________________________________________________________________________    Sounds of                                                                     chords                                                                              3 3 3 5 2 3 2 2 2 3 2 2 2 2 2 2                                         Distortion                                                                    of treble                                                                           5 5 3 5 2 2 1 2 3 2 2 3 2 2 3 3                                         Beats of                                                                      guitar                                                                              2 3 3 3 3 2 3 2 2 3 2 2 2 2 2 2                                         Presence                                                                            3 2 3 2 2 2 1 2 1 2 1 2 2 2 2 1                                         Overall                                                                       evaluation                                                                          65                                                                              65                                                                              60                                                                              75                                                                              45                                                                              45                                                                              35                                                                              40                                                                              40                                                                              50                                                                              35                                                                              45                                                                              40                                                                              40                                                                              45                                                                              35                                        Average                                                                             66      41      42      40                                              __________________________________________________________________________

(2) Howling test

Likewise the tone quality test, the listeners A, B, C and D wereinstructed to hear the three sorts of records mentioned above anddetermine the presence or absence of howling on the basis of thefollowing definition. The pre-main amplifier was employed with bothtreble and bass being located at "flat", while volume pointed toward 11o'clock.

Definition of howling: Howling means all that the tone quality ofreproduced sounds is deteriorated due to external vibrations and thereproduction of source sounds (for example, human voices per se in thecase of vocal) is adversely affected.

Howling was evaluated according to the 10-point system. "10 points"indicated that no howling was observed and source sounds were reproducedas they were. Decrease in points indicated increase in howling and 1point indicates that howling was most severe and normal reproducedsounds were not absolutely obtained. The results are shown in Table 10,wherein "obverse" and "reverse" with respect to the rubber sheets ofExamples 1 to 4 means the following matters:

Obverse: The surface as illustrated in FIG. 1 [i.e., the surfaceprovided with the projections (3) thereon] is regarded as a top surface(i.e., the surface being in contact with the disc record).

Reverse: The surface as illustrated in FIG. 2 [i.e., The surfaceprovided with the grooves (7)] is regarded as a top surface.

Points in Table 10 show the average value of the points evaluated by thefour listeners (fractions were rounded off).

                                      TABLE 10                                    __________________________________________________________________________             Ex. 1                                                                              Ex. 2                                                                             Ex. 3                                                                             Ex. 4                                                                            Com.                                                                             Com.                                                                             Com.                                                                             Com.                                        Record   O R O R O R O R Ex. 1                                                                            Ex. 2                                                                            Ex. 3                                                                            Ex. 4                                       __________________________________________________________________________    Female vocal                                                                           10                                                                              10                                                                              9 10                                                                              9 9 9 9 5  4  4  4                                           Concerto 9 9 8 9 8 8 8 9 4  4  3  4                                           Flamenco guitar                                                                        9 9 8 8 8 8 8 8 4  4  3  4                                           __________________________________________________________________________     O: Obverse                                                                    R: Reverse                                                               

In the above tone quality test and howling test, the rubber sheets ofthe present invention (Examples 1 to 4) gave outstandingly excellentresults in comparison with the conventional rubber sheets (ComparativeExamples 1 to 4). It is believed that the specific physical propertiesand further the specific configuration of the rubber sheet of thepresent invention contributed to such results. That is, the rubber sheetof the present invention is featured by a superlow hardness, i.e. ahardness of from not more than 30°, preferably not more than 20° in Ahardness to not less than 15°, preferably not less than 30° in Fhardness and a high elasticity, i.e. an impact resilience of not lessthan 40%, preferably not less than 50%. The rubber sheet of the presentinvention possesses the specific characteristics of tightly contactingto the disc record or the turntable and preventing slip or resonance, aswill be understood from the results of slip tests and lift testsdiscussed below, and further insuring decrease in the contact area andproviding breathing behavior by use of the specific configuration withthe projections. As a result of the combination of those excellentcharacteristics, the rubber sheet effectively prevents the tone qualityof reproduced sounds from deteriorating due to howling or externalvibrations and permits reproducing source sounds as faithfully as theyare. In the case of the conventional rubber sheets, on the other hand,the rubber sheet of Comparative Example 1 is low in hardness, i.e. 24°in A hardness but low in impact resilience, i.e. 19%, and the rubbersheets of Comparative Examples 2 to 4 are high in hardness. Allconventional rubber sheets, therefore, neither meet the hardness andimpact resilience defined in the present invention nor preventsufficiently the tone quality of reproduced sounds from deterioratingdue to howling or external vibrations.

(3) Slip test

After the rubber sheet was set on the turntable of a player (PL-380 madeby Pioneer Electronic Corporation) together with a disc record and wasrotated at a rate of 331/3 r.p.m. with lowering a stylus on the discrecord, the disk record was urged to stop by gripping lightly the bothopposed radial edges of the disc record by both hands, to thereby allowthe operator to observe the stopping movement of the turntable. It wasregarded that the rubber sheet was "qualified" if the turntable came toa stop immediately after or within 1 second from interruption ofrotating the record and was "disqualified" if it took more than 2seconds for the turntable to come to a stop after interruption ofrotating the record. The results thereof are shown in Table 11.

                  TABLE 11                                                        ______________________________________                                        Rubber sheet      Evaluation                                                  ______________________________________                                        Example 1    Obverse  Qualified                                                            Reverse  "                                                       Example 2    Obverse  "                                                                    Reverse  "                                                       Example 3    Obverse  "                                                                    Reverse  "                                                       Example 4    Obverse  "                                                                    Reverse  "                                                       Comparative Example 1                                                                           Disqualified                                                Comparative Example 2                                                                           "                                                           Comparative Example 3                                                                           "                                                           Comparative Example 4                                                                           "                                                           ______________________________________                                    

In the above test, the rubber sheets of Examples 1 to 4 were allqualified, whereas even the rubber sheet of Comparative Example 1(subjected to no halogenation treatment) which apparently had thehighest frictional resistance among the rubber sheets of ComparativeExamples 1 to 4 was disqualified.

(4) Lift test

This test was conducted to evaluate the tight-contacting properties ofrubber sheets.

The rubber sheet to be lifted [the rubber sheet of Comparative Example 3(weight: 340 g.) was used] was placed on a desk after its center shaftopening was sealed with a cellophane tape, and a rubber sheet to betested was superimposed thereon with the center shaft holes aligned.Then, with the test rubber sheet gripped by thumb and index finger atits center shaft opening so as to stop up the opening, the sheets as aunit were slid across the desk top beyond the edge of the desk intosuspension to observe whether the rubber sheet to be lifted would falloff or not. Since the top surface of the rubber sheet of ComparativeExample 3 to be lifted was already subjected to halogenation treatment,the above test was carried out to study the influence of thehalogenation treatment, both in the case that the test rubber sheet wasattached to the top surface of the rubber sheet of Comparative Example 3to be lifted and in the case that the test rubber sheet was attached tothe untreated bottom surface of the rubber sheet of Comparative Example3. The rubber sheet of Example 1 and the rubber sheet of ComparativeExample 1 (which was deemed as bearing the best tight-contactingproperties out of Comparative Examples 1 to 4) were used as a rubbersheet to be tested. In the case of the rubber sheet of Example 1, thesurface as shown in FIG. 2 was held in contact with the rubber sheet tobe lifted and in the case of the rubber sheet of Comparative Example 1,the flat surface (bottom surface) was held in contact with the rubbersheet to be lifted.

As a result, while the rubber sheet of Example 1 attracted tightly therubber sheet to be lifted as a sucker and lifted up it and thus therubber sheet did not fall, the rubber sheet of Comparative Example 1absolutely failed to attract the rubber sheet to be lifted and it fell.

The above test made clear that the rubber sheet of Example 1 was capableof lifting up the rubber sheet of Comparative Example 3 with 340 g.weight. Next, it was determined what weight in gm. the rubber sheet ofExample 1 was capable of lifting up.

A plurality of the rubber sheets of Comparative Example 3 were stackedwith one on the other and adhered by use of a cellophane tape. Then, therubber sheet of Example 1 was mounted thereon and the lift test wascarried out in the same manner as in the above test.

As a result, it was revealed that the rubber sheet of Example 1 was ableto lift up a pile of 10 rubber sheets of Comparative Example 3 (weight:3.4 kg.). When an attempt was made to lift up the pile of 10 rubbersheets of Comparative Example 3, a break was developed at a portion ofthe rubber sheet of Example 1 where was held by the operator andtherefore the test could be conducted no longer.

As is clear from the results of the above slip and lift tests, therubber sheet of the present invention possesses a high frictionresistance and an excellent tight-contacting property and also possessesthe function of a stabilizer. In case of employing the rubber sheet ofthe present invention, therefore, there is no need for such astabilizer.

(5) Scratch test

This test was conducted on the rubber sheet of Example 1 and the rubbersheet of Comparative Example 3 (which was subjected to halogenationtreatment).

A cartridge portion of a player (SF-135 made by Matsushita ElectricIndustrial Co., Ltd.) was provided with a weight and covered with akraft paper so that a load of about 30 g. was applied to the rubbersheet on the turntable with a contact area of about 2 cm². The cartridgewith the weight was located about 5 cm. inside the periphery of therubber sheet and the turntable was rotated at a rate of 45 r.p.m. Theturntable was braked to stop every one minute to observe if any scratchmark was made.

As a result, in the case of the rubber sheet of Example 1, faint markswere made after 30 minutes but those marks disappeared upon once rubbingwith a finger without leaving any trace. On the other hand, in the caseof the rubber sheet of Comparative Example 3, an infinite number of finescratch marks, though not very conspicuous, were made after 3 minutesand those marks could not be removed by rubbing with a finger. Further,rubbing marks were severely made after 30 minutes and those marks couldnot be also removed by rubbing with a finger.

The conventional rubber sheets are mostly subjected to halogenationtreatment to increase surface hardness in order to prevent the sheetsurface from being marred as in the case of the rubber sheet ofComparative Example 3. As is evident from the foregoing results, eventhough subjected to no halogenation treatment, the rubber sheet of thepresent invention is less marred than the conventional rubber sheetssubjected to halogenation treatment. In addition, in the case of arubber sheet of the present invention, scratch marks, as far as they arefaint, can be easily cleared by rubbing with a finger.

(6) Dust attachment test

Ashes on the burned tip of 10 mm. long of a cigarette were dropped on asheet of paper, crushed by a finger and put on the test rubber sheet 2cm. away from its center. The rubber sheet was held up at a level byboth hands and then, one of the hands is left off so that the rubbersheet hung down. Using the index finger, the sheet was given 3 taps witha constant pressure from the reverse side of the sheet at the portionwhere the ashes were dropped.

As a result, the ashes could be almost completely tapped off in the caseof the rubber sheets of Examples 1 to 4. In the case of the rubbersheets of Comparative Examples 1 to 4, however, the ashes remained inthe shape of a fan with the original position of ashes as its apex. Moreparticularly, the amount of the ashes retained was much in the case ofthe non-halogenated rubber sheet of Comparative Example 1.

It is apparent from the foregoing results that dust is less liable toattach on the rubber sheet of the present invention than on theconventional rubber sheets (Comparative Examples 2 to 4) which aresubjected to halogenation treatment in order to lessen the possibilityof attachment of dust.

Examples 5 to 8

In the same manner as in Examples 1 to 4 except that the rubbercompositions were changed to those shown in Table 12, rubber sheets forturntable were produced.

The physical constants of the resultant rubber sheets were measured inthe same manner as in Examples 1 to 4. The results are also shown inTable 12.

                  TABLE 12                                                        ______________________________________                                        Examples         5       6       7     8                                      ______________________________________                                        Rubber Composition (in parts)                                                 Norsorex (described above)                                                                     80      75      80    100                                    Nipol SBR 1712 (Note 1)                                                                        20      --      --    --                                     Esplene 505 (Note 2)                                                                           --      25      --    --                                     Nipol IR 2200 (Note 3)                                                                         --      --      20    --                                     White factice S  420     330     320   210                                    Sunthene 255     560     450     440   320                                    Sunpar 110 (Note 4)                                                                            --      --      --    30                                     Sundex 790 (Note 5)                                                                            --      --      --    40                                     Dioctyl phthalate                                                                              --      80      --    50                                     FEF carbon       40      80      --    40                                     HAF carbon       --      --      40    --                                     MT carbon        --      --      --    20                                     Zinc oxide       5       5       5     5                                      Stearic acid     1       1       1     1                                      Sumilizer MDP    1       1       1     1                                      Sulfur           2.5     2.5     2.5   2.5                                    Sunceller CZ     10      10      10    9                                      Physical constants                                                            A hardness (degree)                                                                            5       7       5     8                                      Impact resilience (%)                                                                          67      55      59    54                                     Tensile strength (kg./cm..sup.2)                                                               2       5       9     8                                      Elongation (%)   161     155     257   402                                    Specific gravity 0.945   0.979   0.959 1.015                                  ______________________________________                                         (Note 1): Styrenebutadiene rubber made by Nippon Zeon Co., Ltd.               (Note 2): Ethylenepropylene-diene rubber made by Sumitomo Chemical Co.,       Ltd.                                                                          (Note 3): Isoprene rubber made by Nippon Zeon Co., Ltd.                       (Note 4): Paraffinic oil made by Japan Sunoil Co., Ltd.                       (Note 5): Aromatic oil made by Japan Sunoil Co., Ltd.                    

The rubber sheets obtained in Examples 5 to 8 also permitted reproducingexcellent tone quality sounds likewise the rubber sheets obtained inExamples 1 to 4.

Examples 9 to 10

In the same manner as in Examples 1 to 4 except that the rubbercompositions were changed to those shown in Table 13, rubber sheets forturntable were produced. The physical constants of the resulting rubbersheets were measured in the same manner as in Examples 1 to 4. Theresults are also shown in Table 13.

                  TABLE 13                                                        ______________________________________                                        Examples             9       10                                               ______________________________________                                        Rubber Composition (in parts)                                                 Norsorex 150 NA      250     250                                              DOG factice F 10 (Note 1)                                                                          240     300                                              Sunthene 255         130     120                                              Dioctyl phthalate    10      --                                               SRF carbon           40      --                                               MT carbon            --      35                                               Zinc oxide           5       5                                                Stearic acid         1       1                                                Antioxidant DDA (Note 2)                                                                           1       1                                                Suntight S (Note 3)  1       1                                                Sulfur               2       2                                                Sunceller CZ         8       6                                                Physical constants                                                            A hardness (degree)  18      23                                               Impact resilience (%)                                                                              71      73                                               Tensile strength (kg./cm..sup.2)                                                                   21      16                                               Elongation (%)       283     293                                              Specific gravity     0.997   0.999                                            ______________________________________                                         (Note 1): Amber sulfur factice made by D.O.G. Deutsche Oelfabrick Ges. f.     Chem. Erz. mbh & Co.                                                          (Note 2): Diphenylamine antioxidant made by Bayer A.G.                        (Note 3): Microcrystalline wax made by Seiko Kagaku Kabushiki Kaisha     

In the case that a glass sheet of about 300 g. weight was mounted on therespective rubber sheets of Examples 9 to 10, contact area occupiedabout 0.01% of the entire area of one surface of the rubber sheet.

The following tests were conducted on the rubber sheets of Examples 9and 10.

(1) Tone quality test

This test was conducted in the same manner as in Examples 1 to 4 exceptthat the test record and parameters were changed as follows: The resultsare shown in Tables 14 and 15 together with the results obtained on therubber sheets of Example 4 and Comparative Examples 1 to 4.

Test record

Title of record: Bach: toccata and fugue, E. Power Biggs at the ThomasChurch (CBS Sony Inc.)

Performer: E. Power Biggs (organ)

Program: Toccata and fugue in D minor

Parameters

(a) if peculiar sounds of a pipe organ are reproduced

(b) if treble is clear

(c) if bass is solemn

(d) if a melody is definitely separate from an accompaniment

                                      TABLE 14                                    __________________________________________________________________________            Example 9    Example 10                                                                               Example 4                                     Parameters                                                                            A  B  C  D  A  B  C D  A B C D                                        __________________________________________________________________________    Sounds of a                                                                   pipe organ                                                                            5  5  5  5  5  5  4 5  4 4 3 4                                        Treble  5  5  5  5  5  5  4 5  5 4 5 5                                        Bass    5  5  5  5  5  5  5 5  4 5 4 5                                        Melody &                                                                      accompaniment                                                                         5  5  5  5  5  5  5 5  5 5 5 5                                        Overall                                                                       evaluation                                                                            100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              90                                                                              100                                                                              90                                                                              90                                                                              85                                                                              95                                       Average 100         98         90                                             __________________________________________________________________________

                                      TABLE 15                                    __________________________________________________________________________            Comparative                                                                           Comparative                                                                           Comparative                                                                           Comparative                                           Example 1                                                                             Example 2                                                                             Example 3                                                                             Example 4                                     Parameters                                                                            A B C D A B C D A B C D A B C D                                       __________________________________________________________________________    Sounds of a                                                                   pipe organ                                                                            2 3 3 2 2 2 2 2 1 2 1 2 1 1 2 2                                       Treble  2 2 3 3 2 3 2 2 2 1 2 2 2 1 2 1                                       Bass    2 3 2 3 1 2 2 1 2 1 2 2 1 2 3 1                                       Melody &                                                                      accompaniment                                                                         2 1 2 2 1 1 1 1 1 1 1 1 1 1 1 1                                       Overall                                                                       evaluation                                                                            40                                                                              45                                                                              50                                                                              50                                                                              30                                                                              40                                                                              35                                                                              30                                                                              30                                                                              25                                                                              30                                                                              35                                                                              25                                                                              25                                                                              40                                                                              25                                      Average 46      34      30      29                                            __________________________________________________________________________

(2) Howling test

The test was conducted in the same manner as in Examples 1 to 4 exceptthat the test record was changed to that used in the above tone qualitytest and the test was carried out only in the case that the rubber sheetwas mounted with the surface shown in FIG. 1 [the surface provided withthe projections (3)] in contact with the record. The results are shownin Table 16 together with the results obtained on the rubber sheets ofExample 4 and Comparative Examples 1 to 4.

                  TABLE 16                                                        ______________________________________                                                         Howling                                                      ______________________________________                                        Examples         9     10                                                                      10    10                                                                      4     10                                                     Comparative Example                                                                            1     3                                                        "              2     2                                                        "              3     3                                                        "              4     2                                                      ______________________________________                                    

In the above tone quality and howling tests, the rubber sheets of thepresent invention (Examples 9 to 10) brought about outstandinglyexcellent results as compared with the conventional rubber sheets(Comparative Examples 1 to 4).

The rubber sheets of Examples 9 and 10 with A hardnesses of 18° and 23°,respectively, helped reproduce much better sounding sounds than therubber sheet of Example 4 with a lower hardness of 9° in A hardness.

(3) Slip test

This test was carried out in the same manner as in Examples 1 to 4. As aresult, the rubber sheets of Examples 9 and 10 were both qualified.

(4) Lift test

This test was carried out on the rubber sheet of Example 9 in the samemanner as in Examples 1 to 4. As a result, the rubber sheet of Example 9lifted up to 10 sheets (weight: 3.4 kg.) of the rubber sheet ofComparative Example 3.

(5) Scratch test

This test was carried out on the rubber sheet of Example 9 in the samemanner as in Examples 1 to 4. As a result, faint marks were made on therubber sheet of Example 9 after 30 min. but removed upon once rubbing byfinger without leaving any trace.

(6) Dust attachment test

The test was carried out in the same manner as in Examples 1 to 4. As aresult, substantially no ashes remained on the rubber sheets of Examples9 and 10.

Examples 11 and 12

In the same manner as in Examples 1 to 4 except that the rubbercompositions were changed to those shown in Table 17, rubber sheets forturntable were produced.

The physical constants of the resulting rubber sheets were measured inthe same manner as in Examples 1 to 4. The results are shown in Table18.

                  TABLE 17                                                        ______________________________________                                        Rubber composition (in parts)                                                                 Examples                                                      Components        11      12                                                  ______________________________________                                        Norsorex 150 NA   250     250                                                 DOG factice F 10  230     200                                                 Sunthene 255      400     --                                                  Sundex 790        --      1,000                                               Rapeseed oil      --      15                                                  Dioctyl phthalate 50      35                                                  FEF carbon        50      --                                                  MT carbon         --      50                                                  Zinc oxide        5       5                                                   Stearic acid      1       1                                                   Antioxidant DDA   2       2                                                   Suntight S        1       1                                                   Sulfur            2.5     2.5                                                 Sunceller CZ      8       10                                                  ______________________________________                                    

                                      TABLE 18                                    __________________________________________________________________________                     Impact                                                                             Tensile                                                 Hardness (degree)                                                                              resilience                                                                         strength                                                                            Elongation                                                                          Specific                                    Example                                                                            C hardness                                                                          F hardness                                                                          (%)  (kg./cm..sup.2)                                                                     (%)   gravity                                     __________________________________________________________________________    11   11    73    63   6.5   387   0.977                                       12   --    31    51   0.3   490   1.004                                       __________________________________________________________________________

The following tests were effected on the rubber sheets of Examples 11and 12.

(1) Tone quality test

The test was carried out in the same manner as in Examples 1 to 4 exceptthat the test record and parameters were changed as follows: The resultsare shown in Tables 19 and 20 together with the results obtained on therubber sheets of Example 2 and Comparative Examples 1 to 4.

Test record

Title of record: "1812 Overture" by Tchaikovsky (Telarc Records)

Performer: The Cincinnati Symphony Orchestra

Conductor: Erich Kunzel

Parameters

(a) if sounds of a cannon are well reproduced

(b) if treble is not distorted

(c) if bass is well reproduced

(d) if the impressions of presence are sufficient

                  TABLE 19                                                        ______________________________________                                        Para-  Example 11     Example 12  Example 2                                   meters A      B     C    D   A   B   C   D   A   B   C                                                     D                                                ______________________________________                                        Sounds                                                                                                     of a                                                                          cannon 5 5 5 4 4 5 4 5 4 4 4 3                                                Distor-                                                                       tion of                                                                       treble 5 4 5 5 5 4 5 5 5 4 5 5                                                Bass 5 5 5 5 5 5 5 4 4 5 5 5                                                  Presence 5 5 5 5 5 5 5 4 5 4 5 5                                              Overall                                                                       evalu-                                                                        ation 100 95 100 95 95 95 95 90 90 85 95 90      Average                                                                              98            94          90                                           ______________________________________                                    

                                      TABLE 20                                    __________________________________________________________________________    Comparative   Comparative                                                                           Comparative                                                                           Comparative                                     Example 1     Example 2                                                                             Example 3                                                                             Example 4                                       Parameters                                                                          A B C D A B C D A B C D A B C D                                         __________________________________________________________________________    Sounds of a                                                                   cannon                                                                              2 2 2 2 2 1 2 2 1 2 2 1 1 2 2 2                                         Distortion                                                                    of treble                                                                           2 1 2 2 2 2 3 2 2 1 2 3 1 2 2 2                                         Bass  3 3 3 3 3 2 3 3 2 2 3 2 2 3 3 2                                         Presence                                                                            3 4 2 4 2 2 1 2 3 2 3 2 3 2 2 2                                         Overall                                                                       evaluation                                                                          50                                                                              50                                                                              45                                                                              55                                                                              45                                                                              35                                                                              45                                                                              45                                                                              40                                                                              35                                                                              50                                                                              40                                                                              35                                                                              45                                                                              45                                                                              40                                        Average                                                                             50      43      41      41                                              __________________________________________________________________________

(2) Howling test

The test was conducted in the same manner as Examples 1 to 4 except thatthe test record was changed to that used in the above tone quality testand the test was carried out only in the case that the rubber sheet wasmounted with the surface shown in FIG. 1 [the surface provided with theprojections (3)] in contact with the record. The results are shown inTable 21 together with the results obtained on the rubber sheets ofExample 2 and Comparative Examples 1 to 4.

                  TABLE 21                                                        ______________________________________                                                         Howling                                                      ______________________________________                                        Examples         11    10                                                                      12    9                                                                       2     8                                                      Comparative Example                                                                            1     4                                                        "              2     3                                                        "              3     3                                                        "              4     2                                                      ______________________________________                                    

In the above tone quality and howling tests, the rubber sheets of thepresent invention (Examples 11 and 12) brought about outstandinglyexcellent results as compared with the conventional rubber sheets(Comparative Examples 1 to 4).

The rubber sheets of Examples 11 and 12 with C hardness of 11° (Fhardness of 73°) and F hardness of 31°, respectively, further preventedhowling and helped reproduce sounds of superlow frequencies as comparedwith the rubber sheet of Example 2 with a higher hardness of 5° in Ahardness.

(3) Slip test

This test was carried out in the same manner as in Examples 1 to 4. As aresult, the rubber sheets of Examples 11 and 12 were both qualified.

(4) Scratch test

This test was carried out on the rubber sheet of Example 11 in the samemanner as in Examples 1 to 4. As a result, faint marks were made after30 min. but removed upon once rubbing by finger without leaving anytrace.

(5) Dust attachment test

This test was carried out in the same manner as in Examples 1 to 4. As aresult, substantially no ashes remained on the rubber sheets of Examples11 and 12.

Examples 13 to 15

In the same manner as in Examples 1 to 4 except that the rubbercompositions were changed to those shown in Table 22, rubber sheets forturntable were produced.

The physical constants of the resulting rubber sheets were measured inthe same manner as in Examples 1 to 4. The results are shown in Table23.

                  TABLE 22                                                        ______________________________________                                        Rubber composition (in parts)                                                                  Examples                                                     Components         13      14        15                                       ______________________________________                                        Nipol SBR 1712     137.5   --        --                                       Natural rubber     --      100       --                                       Baypren 112 (Note 1)                                                                             --      --        100                                      DOG factice F 10   200     250       --                                       DOG factice NP 17 (Note 2)                                                                       --      --        240                                      Sunthene 255       200     110       200                                      MT carbon          20      50        45                                       Zinc oxide         5       5         --                                       Magnesia           --      --        4                                        Stearic acid       1       1         1                                        Antioxidant DDA    2       1         1                                        Suntight S         1       1         1                                        Sulfur             2       2         2                                        Sunceller CZ       4       --        --                                       Nocceler DM (Note 3)                                                                             --      2.7       --                                       Nocceler D (Note 4)                                                                              --      0.7       --                                       Nocceler TT (Note 5)                                                                             --      0.4       --                                       Nocceler DT (Note 6)                                                                             --      --        1.1                                      Nocceler TS (Note 7)                                                                             --      --        1.1                                      ______________________________________                                         (Note 1): Chloroprene rubber made by Bayer A.G.                               (Note 2): Amber sulfur factice made by D.O.G. Deutsche Oelfabrick Ges. f.     Chem. Erz. mbh & Co.                                                          (Note 3): Curing accelerator made by Ouchi Chemical Industry Co., Ltd.        (Note 4): Curing accelerator made by Ouchi Chemical Industry Co., Ltd.        (Note 5): Curing accelerator made by Ouchi Chemical Industry Co., Ltd.        (Note 6): Curing accelerator made by Ouchi Chemical Industry Co., Ltd.        (Note 7): Curing accelerator made by Ouchi Chemical Industry Co., Ltd.   

                                      TABLE 23                                    __________________________________________________________________________    Hardness (degree)   Impact                                                                             Tensile                                                                             Elonga-                                             A hard-                                                                            C hard-                                                                            F hard-                                                                            resilience                                                                         strength                                                                            tion Specific                                  Example                                                                            ness ness ness (%)  (kg./cm..sup.2)                                                                     (%)  gravity                                   __________________________________________________________________________    13    9   30   --   62   8     258  1.009                                     14   23   51   --   62   5.4   223  0.984                                     15   --   10   69   45   3.6   322  1.026                                     __________________________________________________________________________

The following tests were effected on the rubber sheets of Examples 13 to15.

(1) Tone quality test

This test was carried out in the same manner as in Examples 1 to 4. Theresults are shown in Tables 24 to 26.

                  TABLE 24                                                        ______________________________________                                        (Female vocal)                                                                Example 13        Example 14  Example 15                                      Parameters                                                                            A      B     C   D   A   B   C   D   A   B   C                                                     D                                                ______________________________________                                        Nearness to                                                                                                human                                                                         voices 5 4 4 5 4 5 4 5 4 5 4 4                                                Distortion                                                                    of treble 5 5 5 4 4 5 5 4 5 4 5 5                                             Accom-                                                                        panying                                                                       guitar 5 5 5 5 5 5 4 5 4 5 5 5                                                Presence 5 5 4 5 5 4 5 5 5 5 4 5                                              Overall                                                                       evaluation 100 95 90 95 90 95 90 95 90 95 90                                  95                                               Average 95           93          93                                           ______________________________________                                    

                  TABLE 25                                                        ______________________________________                                        (Concerto)                                                                    Para-  Example 13    Example 14   Example 15                                  meters A     B     C    D   A    B   C   D   A   B   CD                       ______________________________________                                        Sounds                                                                        of chords                                                                            5     5     5    5   5    5   5   4   4   5   5  4                     Distor-                                                                       tion of                                                                       treble 5     4     5    5   5    5   5   5   5   5   5  5                     Bass   4     5     5    4   5    4   5   4   5   4   5  5                     Presence                                                                             5     5     5    5   5    5   4   5   5   4   5  5                     Overall                                                                       evalu-                                                                        ation  95    95    100  95  100  95  95  90  95  90  100 95                   Average                                                                              96           95           95                                           ______________________________________                                    

                  TABLE 26                                                        ______________________________________                                        (Flamenco guitar)                                                             Para-  Example 13    Example 14   Example 15                                  meters A     B     C    D   A    B   C   D   A   B   C                                                     D                                                ______________________________________                                        Sounds                                                                                                     of chords 5 5 5 4 5 4 5 5 5 4 4 4                                             Distor-                                                                       tion of                                                                       treble 5 4 5 5 5 5 4 5 5 5 5 5                                                Beats of                                                                      guitar 4 5 5 5 5 5 5 4 4 5 5 4                                                Presence 5 5 5 5 5 5 5 5 5 5 5 5                                              Overall                                                                       evalu-                                                                        ation 95 95 100 95 100 95 95 95 95 95 95 90      Average                                                                              96           96           94                                           ______________________________________                                    

(2) Howling test

The test was conducted in the same manner as in Examples 1 to 4 exceptthat the test was carried out only in the case that the rubber sheet wasmounted with the surface shown FIG. 1 [the surface provided with theprojections (3)] in contact with the record. The results thereof areshown in Table 27.

                  TABLE 27                                                        ______________________________________                                                    Examples                                                          Record        13          14     15                                           ______________________________________                                        Female vocal  8           8      8                                            Concerto      9           8      8                                            Flamenco guitar                                                                             9           8      8                                            ______________________________________                                    

In the above tone quality and howling tests, the rubber sheets of thepresent invention (Examples 13 to 15) brought about outstandinglyexcellent effects as compared with the conventional rubber sheets[Comparative Examples 1 to 4 (see Tables 5, 7, 9 and 10)].

(3) Slip test

This test was carried out in the same manner as in Examples 1 to 4. As aresult, all of the rubber sheets of Examples 13 to 15 were qualified.

(4) Lift test

This test was carried out on the rubber sheet of Example 13 in the samemanner as in Examples 1 to 4. As a result, the rubber sheet of Example13 lifted up to 6 sheets (weight: 2.0 kg.) of the rubber sheet ofComparative Example 3.

(5) Scratch test

This test was carried out on the rubber sheet of Example 13 in the samemanner as in Examples 1 to 4. As a result, faint marks were made after30 min. but removed upon once rubbing by finger.

(6) Dust attachment test

This test was carried out in the same manner as in Examples 1 to 4. As aresult, substantially no ashes remained on the rubber sheets of Examples13 to 15.

Examples 16 to 19

In the same manner as in Examples 1 to 4 except that the rubbercompositions were changed to those shown in Table 28, rubber sheets forturntable were produced.

The physical constants of the resulting rubber sheets were measured inthe same manner as in Examples 1 to 4. The results are shown in Table29.

                  TABLE 28                                                        ______________________________________                                        Rubber composition (in parts)                                                              Examples                                                         Components     16      17       18    19                                      ______________________________________                                        Nipol SBR 1712 137.5   --       --    --                                      Nipol IR 2200  --      100      --    --                                      Nordel 1040 (Note 1)                                                                         --      --       100   --                                      Millathane 76 (Note 2)                                                                       --      --       --    100                                     DOG factice F 10                                                                             230     200      250   --                                      DOG factice NP 17                                                                            --      --       --    200                                     DOG factice DS SOFT                                                                          --      15       --    --                                      (Note 3)                                                                      Sunthene 255   200     15       200   250                                     Sunpar 110     --      70       --    --                                      Thiokol TP-95 (Note 4)                                                                       --      --       --    10                                      SRF carbon     --      5        --    --                                      MT carbon      60      --       50    --                                      Light calcium carbonate                                                                      --      --       --    10                                      Zinc oxide     5       5        5     --                                      Zinc stearate  --      --       --    0.5                                     Stearic acid   1       1        1     --                                      Antioxidant DDA                                                                              2       1        1     --                                      Suntight S     1       1        1     --                                      Sulfur         2       2.2      2.5   1.5                                     Sunceller CZ   4       2        --    --                                      Nocceler DM    --      --       --    3                                       Nocceler TT    --      --       1.2   --                                      Nocceler M (Note 5)                                                                          --      --       2     2                                       Nocceler TRA (Note 6)                                                                        --      --       1.2   --                                      Thiokol ZC 456 (Note 7)                                                                      --      --       --    1                                       ______________________________________                                         (Note 1): Ethylenepropylene-diene copolymer made by E. I. Du Pont de          Nemours & Co.                                                                 (Note 2): Urethane rubber made by Technical Sales & Engineering Inc.          (Note 3): Amber soft sulfur factice made by D.O.G. Deutsche Oelfabrick        Ges. f. Chem. Erz. mbh & Co.                                                  (Note 4): Plasticizer made by Thiokol Corp.                                   (Note 5): Curing accelerator made by Ouchi Chemical Industry Co., Ltd.        (Note 6): Curing accelerator made by Ouchi Chemical Industry Co., Ltd.        (Note 7): Curing accelerator made by Thiokol Corp.                       

                                      TABLE 29                                    __________________________________________________________________________                     Impact                                                                             Tensile                                                 Hardness (degree)                                                                              resilience                                                                         strength                                                                            Elongation                                                                          Specific                                    Example                                                                            A hardness                                                                          C hardness                                                                          (%)  (kg./cm..sup.2)                                                                     (%)   gravity                                     __________________________________________________________________________    16   18    46    59   21    364   1.016                                       17   8     28    64   6     330   0.921                                       18   6     25    52   7     241   0.971                                       19   4     22    61   2     182   1.008                                       __________________________________________________________________________

Likewise the rubber sheets of Examples 13 to 15, the rubber sheets ofExamples 16 to 19 permitted reproducing excellent tone quality sounds.

I claim:
 1. A rubber disc for a record player turntable which is asheet-like cured molding of a rubber composition, said cured moldinghaving a hardness within the range from 30° as measured with an A-typerubber hardness tester to 15° as measured with an F-type rubber hardnesstester and an impact resilience of not less than 40%.
 2. The rubber discof claim 1, wherein the hardness is not more than 20° as measured withthe A-type rubber hardness tester and not less than 30° as measured withthe F-type rubber hardness tester and the impact resilience is not lessthan 50%.
 3. The rubber disc of claim 1, wherein the hardness is morethan 15° as measured with the A-type rubber hardness tester and not morethan 30° as measured with the A-type rubber hardness tester and theimpact resilience is not less than 50%.
 4. The rubber disc of claim 1,wherein the hardness is from 1° to 15° as measured with the A-typerubber hardnesss tester.
 5. The rubber disc of claim 1, wherein thehardness is less than 1° as measured with the A-type rubber hardnesstester and not less than 15° as measured with the F-type rubber hardnesstester.
 6. The rubber disc of any one of claims 1, 2, 3, 4 and 5,wherein the cured molding is a cured molding of a rubber compositioncontaining predominantly polynorbornene as a rubber component.
 7. Therubber disc of any one of claims 1, 2, 3, 4 and 5, wherein the curedmolding is a cured molding of a rubber composition containingpredominantly as a rubber component at least one member selected fromthe group consisting of natural rubber, isoprene rubber, chloroprenerubber, styrene-butadiene rubber, butadiene rubber, butyl rubber,ethylene-propylene rubber, ethylene-propylene-diene rubber, nitrilerubber, acryl rubber, urethane rubber, chlorinated polyethylene,chlorosulfonated polyethylene, epichlorohydrin rubber, polysulfiderubber and silicone rubber.
 8. The rubber disc of claim 1, wherein therubber composition comprises (A) 100 parts by weight of a rubbercomponent, (B) 1 to 2,000 parts by weight of a factice and (C) 20 to2,000 parts by weight of a softening agent.
 9. The rubber disc of claim8, wherein the amount of the component (B) is from 5 to 2,000 parts byweight and the amount of the component (C) is from 50 to 2,000 parts byweight.
 10. The rubber disc of claim 9, wherein the amount of thecomponent (B) is from 100 to 1,500 parts by weight and the amount of thecomponent (C) is from 200 to 1,500 parts by weight.
 11. The rubber discof any one of claims 8, 9 and 10, wherein the component (A) comprisespredominantly polynorbornene.
 12. The rubber disc of any one of claims8, 9 and 10, wherein the component (A) comprises predominantly at leastone member selected from the group consisting of natural rubber,isoprene rubber, chloroprene rubber, styrene-butadiene rubber, butadienerubber, butyl rubber, ethylene-propylene rubber,ethylene-propylene-diene rubber, nitrile rubber, acryl rubber, urethanerubber, chlorinated polyethylene, chlorosulfonated polyethylene,epichlorohydrin rubber, polysulfide rubber and silicone rubber.
 13. Therubber disc of claim 1, wherein one surface of the body of the rubbersheet is provided with a large number of projections.
 14. The rubberdisc of claim 13, wherein at least the head of each of the projectionsis ridge-shaped to make substantially a line contact with a turntable ora disc record.
 15. The rubber disc of claim 13, wherein at least thehead of each of the projections is pointed to make substantially a pointcontact with a turntable or a disc record.
 16. The rubber disc of anyone of claims 13, 14 and 15, wherein the height of the projections isfrom 0.1 to 5 mm.
 17. The rubber disc of claim 16, wherein the height ofthe projections is from 0.3 to 1.8 mm.
 18. The rubber disc of claim 13,wherein the projections are closely aligned with one another in aplurality of concentrically circular arrays.
 19. The rubber disc ofclaim 18, wherein at least the head of each of the projections isridge-shaped to make substantially a line contact with a turntable or adisc record.
 20. The rubber disc of claim 19, wherein the ridges of theprojections are aligned along the radical direction of the body of therubber sheet.
 21. The rubber disc of claim 18, wherein at least the headof each of the projections is pointed to make substantially a pointcontact with a turntable or a disc record.
 22. The rubber disc of anyone of claims 18, 19, 20 and 21, wherein the head of each projection inone of the concentrically circular arrays is arranged in correspondencewith a valley between the two adjacent projections in the adjacentconcentrically circular array in the radial direction of the body of therubber sheet.
 23. The rubber disc of claim 22, wherein, when beingmounted with the surface provided with the projections in contact with adisc record and the other surface in contact with a turntable, theprojections are highest in the most inner concentrically circular arrayand lower gradually in the direction toward outer arrays of theprojections and air is confined within a space, over the central portionof the body of the rubber sheet, defined by the most innerconcentrically circular array of the projections and the disc record.24. The rubber disc of claim 23, wherein the surface to be in contactwith the turntable is provided with a plurality of concentricallycircular grooves in correspondence with the plurality of theconcentrically circular arrays of the projections and air is confined atleast in the most inner groove.
 25. The rubber disc of claim 1, which isa sheet-like cured molding of a rubber composition comprising (A) 100parts by weight of polynorbornene alone or a mixture thereof with otherrubber component, (B) 5 to 500 parts by weight of a white factice aloneor a mixture thereof with other factice and (C) 200 to 1,000 parts byweight of at least one oil selected from the group consisting ofaromatic oils, naphthenic oils, paraffinic oils, vegetable oils andanimal oils, said cured molding having a hardness of 1° to 15° asmeasured with the A-type rubber hardness tester, a tensile strength of 1to 50 kg./cm²., an elongation of 120 to 1,000%, an impact resilience of40 to 90% and a specific gravity of 0.93 to 1.05 and one surface of thebody of the rubber sheet being provided with projections with a heightof 0.3 to 1.8 mm. closely aligned with one another in a plurality ofconcentrically circular arrays.